It is a known phenomenon that certain fluorescent materials are capable of absorbing radiated electromagnetic energy in the near ultraviolet spectrum and emitting it at a longer wavelength in the visible spectrum of light. This phenomenon enables various inspection and detection techniques using fluorescent dyes or pigments illuminated by an ultraviolet radiation source that then re-radiate with luminescence in the visible spectrum. For example, a refrigerant soluble fluorescent dye is often used in air conditioning systems to detect slow leaks therein. The fluorescent dye glows at a leak site when bathed in UV light.
Similarly, many paints and coatings may employ fluorescent agents therein to permit the coatings to be inspected once applied to a surface or substrate simply by exposing the coated surface to a UV light source and carefully examining the light pattern emitted therefrom. This technique is often employed where maximal coating integrity over a surface is necessary. For example in naval applications wherein coated components are routinely subjected to salt water and its corrosive effects, it is typically useful to conduct a thorough inspection of all coated components to detect the presence of any voids, imperfections or “holidays” in the applied coating, thereby permitting corrective action prior to placing the component in service.
However, since fluorescence is a fairly weak effect, high intensity UV light sources are required for accurate and reliable holiday detection. Additionally, many areas that must be coated, such as ballast tanks on ships, are not readily accessible or visible and thus are not amenable to a simple process of UV light irradiation and attendant visual inspection. Furthermore, many coated areas requiring inspection have irregular surfaces that require more thorough scrutiny to determine the absence or presence of holidays in the coating. This problem is particularly acute in welded areas that are coated, since the surfaces of weld beads tend to be highly irregular and thus prove difficult to thoroughly coat and inspect. Discontinuities in coatings are often quite small and as such, are not readily visible to the naked eye.
Prior art coating inspection techniques have proven to be unwieldy and inefficient. For example, a wet sponge/conductivity test is often employed wherein a voltage is applied to a wet sponge which is them slowly moved over the entire surface. This test may only be employed where a non-conductive coating is applied to a conductive surface. Where a void or holiday is present, an electrical current will flow through the sponge as electricity is conducted from the void area through the wet sponge. Many wet-sponge testing systems employ other visual or audible indicators to alert an inspector to a coating discontinuity. This holiday detection technique is both time and labor intensive, since the sponge must be moved over the entire coated surface in order to detect any holidays. Furthermore, this inspection technique can only be used be when the coating being inspected is completely cured, thereby delaying inspection of freshly applied coatings.
Some camera-based inspection systems do exist, but these systems suffer from an inability to operate in low-light conditions and often prove slow to operate. Furthermore, prior art systems are costly, bulky and unwieldy, and are unable to discriminate between an absence of fluorescence caused by a void and the complete absence of a surface. Another particular difficulty with prior art inspection systems is the inability to inspect portions of surfaces due to the effects of glare and shadows caused by illumination or incident light.